The present invention relates to printed wiring board capable of forming an electrical connection to an external device.
Conventionally, a printed wiring board is provided with a connecting terminal for forming an electrical connection to an external device, such as an other printed wiring board or other electronic equipment. The electric connection is realized by connecting a connecting terminal to another flexible printed wiring board by thermo-bonding or by inserting the connecting terminal to a connector of external devices.
FIGS. 6 and 7 show a conventional printed wiring board having such construction. An insulating base material 61 is provided having one surface with a predetermined circuit 62 formed by an operation such as screen printing exposure, etching or the like. Reference numeral 63 is a connecting terminal formed on the end portion of the base material simultaneously with the formation of the circuit 62. This connecting, terminal informing an electrical connection to the an external device. A terminal circuit 64 is also connected to the connecting terminal 63. A region including the circuit 62 of the base material 61 and the terminal circuit 64 is coated by a solder resist layer 65, thereby protecting them from oxidation. In this case, the solder resist layer 65 is not coated on a connection terminal forming region 66 for ejecting the electric connection to the external equipment.
In such a construction, the connecting terminal 63 may be subjected to insulation failure due to oxidization and an adhesion strength of thermo-bonding for the electric connection becomes insufficient. Thus, it is conventionally necessary to reinforce the connecting terminal 63. To ,this end, respective connecting terminals 63 are coated with a protecting layer 67. This protecting layer 67 is formed by printing conductive carbon ink on the base material 61. As shown in FIG. 7, a part of this protecting layer 67 is overlapped on the end portion of the previously applied solder resist layer 65.
As circuits becoming more dense, the distance between the connecting terminals 63 formed on the base material 61 becomes more narrow. In the conventional printed wiring board, a short-circuit results between the connecting terminals 63 and insulation becomes deteriorated. In other words, a copper foil, such as the connecting terminal provided on the base material 61, has a thickness of about 35 .mu.m, and solder resist layer 65 has a thickness of 14.about.17 .mu.m, so that stepped level differences of 49.about.52 .mu.m are caused on the surface of the base material 61, and thus the protecting layer 67 must be printed on such a stepped level differences. On printing the protecting layer on the stepped level differences, as shown in FIG. 8, a blur 68 of the protecting layer 67 occurs at the overlapped portion of the protecting layer 67 and the solder resist layer 65. When such a blur 68 occurs on the high densified connecting terminal 63, a bridged connecting is generated between adjacent connection terminals 63, so that the connecting terminals 63 are short-circuited to each other and the insulating property between the connecting terminals 63 becomes inferior.